1. Field of the Invention
The invention relates to an injection mould, for instance for manufacturing disc-like information carriers or plastic lenses.
2. Description of the Prior Art
In moulds for the production of optical information carriers an insert disc or stamper is arranged in the mould. This stamper carries a mechanical pattern which corresponds with the information for arranging on the information carrier to be manufactured. The stamper is arranged against a hardened steel insert piece, often designated as a mirror, and often held by means of a vacuum. The stamper is manufactured by means of an electro-deposition process and usually consists substantially of nickel. For manufacturing a CD the stamper has a diameter of 138 mm and a thickness of 0.3 mm.
During injection moulding in the mould, heated plastic, generally polycarbonate that is plasticized under high pressure, is injected against the stamper when the mould is closed. The injection pressure of the plastic is locally 1500 bar, wherein the temperature of the plasticized plastic is about 320.degree. C.
The stamper placed in the mould is not wholly flat and will be pressed against the mirror under the influence of the high injection pressure. The hot plastic will also cause the stamper, which normally has a temperature of 60.degree. C., to expand. This combination of pressure and expansion ensures that the stamper slides with micro-movements over the highly polished mirror surface. These micro-movements cause so-called cold weld between the hardened mirror surface and the stamper. Owing to this cold weld a connection occurs wherein the nickel adheres to the steel of the mirror. In practice this results after about 500,000 cycles in a mirror surface which is no longer wholly flat and therefore no longer capable of transferring the information structures from the stamper to the plastic information carrier. The result is deformation of this information, which results in rejection of the information carrier.
A known solution for this problem of pit deformation by cold weld is to provide the mirror surface with a thin coating of several micrometers. This known coating consists of TiN and is arranged by means of "Physical Vapour Deposition" or PVD. In respect of a correct pit formation this coating must be of optical quality without the occurrence of local build-up of so-called droplets (large peaks due to droplet formation). These droplets penetrate into the stamper and cause pit deformation. The result of this known coating is in practice as follows:
Reduction of the cold weld. With the coating about 3,000,000 cycles can be performed with a mirror before pit deformation occurs. This means that the lifespan of the mirror is increased by a factor of 6. PA1 Dirt can be cleaned from the mirror surface more easily. PA1 The mirror surface can be cleaned more easily when nickel is deposited thereon. PA1 A harder and more scratch-resistant surface owing to the use of the coating, so that damage, for instance during cleaning, occurs less quickly, which contributes to a longer lifespan. PA1 Less rejection of manufactured information carriers in that the surface of the mirror is contaminated less rapidly with nickel and pit deformation thereby occurs later. PA1 For applying of the coating a high process temperature is required, i.e. 480.degree. C. This may cause a change in shape due to the residual austenite conversion in the hardened steel of the mirror. PA1 Cold weld is still found to occur despite the fact that a substantial improvement in standing time is realized. PA1 A coefficient of friction of the TiN coating in a lubricant-free situation which is roughly equal to that of steel, i.e. 0.5. It is therefore still not easily possible for the stamper to slide over the mirror surface counter to the pressure of the plastic and possibly counter to the pressure of a resilient venting ring. This causes a certain degree of bulging of the stamper which results in pit deformation, particularly on the outer edge of the information carrier. This phenomenon is further enhanced in the production of a Digital Versatile Disc or DVD, wherein the track pitch is 0.8 micrometer compared to a Compact Disc or CD with a track pitch of 0.6 micrometer. PA1 two mould parts mutually movable between a closed and an open position, which mould parts bound in closed position a mould cavity into which in the closed position heated plasticized plastic can be injected by means of supply means, wherein one flat end surface of the mould cavity carries a mechanical pattern either directly or via an insert disc consisting predominantly of nickel, which pattern corresponds with the information for arranging on the information carrier to be manufactured, PA1 wherein the one mould part has a cylindrical inner surface partly bounding the mould cavity and the other mould part has a cylindrical outer surface, which cylindrical surfaces can slide over each other, wherein at least one of these surfaces can form part of an optionally resilient venting ring, PA1 wherein at least the surfaces bounding the mould cavity and said cylindrical surfaces consist of steel, PA1 wherein at least a part of said surfaces is provided with a coating which does not consist of TiN and which reduces the coefficient of friction of the relevant surface to a value which amounts to a maximum of 60% of the value obtainable with a coating of TiN, which coefficient of friction amounts for instance to a maximum of 0.3 with said insert disc consisting predominantly of nickel; PA1 wherein at least said cylindrical inner surface or outer surface is provided with said coating. PA1 About the same hardness of 2300 HV as the known TiN coating. PA1 A considerably reduced coefficient of friction relative to TiN under lubricant-free conditions, i.e. a maximum of about 0.2 compared to 0.5 for TiN under lubricant-free conditions. PA1 The option of applying the coating in droplet-free manner. PA1 A process temperature during application of a maximum of about 160.degree. C. compared to 480.degree. C. for TiN. PA1 The TiN coating showed cold weld between coating and stamper, wherein locally the coating was completely worn away. This was not the case with the DLC coating. PA1 The nickel coating of the nickel-coated ball was completely worn away in the contact with the TiN coating, while this nickel coating remained fully intact when the DLC coating was used. PA1 Owing to the low coefficient of friction the stamper can expand more easily under the high injection pressure of the plastic and thus essentially prevent pit deformation on the outer edge of the disc. PA1 Cold weld occurs later as a result of the low coefficient of friction. The lifespan of the mirror is hereby increased by a factor of 2, as in the case of the stamper with a lifespan of 40,000 cycles on a mirror with TiN coating and 60,000 cycles on a mirror with DLC coating. PA1 Less rejection of discs. Costs and environment are important aspects here. PA1 No deformation of the mirror owing to the low process temperature when the DLC coating is applied. PA1 Easier to de-coat than TiN owing to the composition of the coating and the low de-coat temperature of a maximum of 160.degree. C. PA1 Easier cleaning of the mirror surface owing to less deposition of both dirt and nickel. PA1 More scratch-resistant surface and therefore less immediate damage of this surface. At this moment it is expected that a factor of 2 is attainable. PA1 Little deposition of dirt and precipitation of additives. Cleaning will hereby also become easier. PA1 The so-called scoring or cold weld between the mirror and the resilient venting ring occurs at a much later stage or even not at all. A short-duration test with DLC coating or a venting ring has shown an improvement by a factor of 5. PA1 Owing to the low coefficient of friction of the plastic over the surface provided with a coating the stress level in the disc will be reduced, which results in less diffusion of the scanning laser beam during reading of the disc. PA1 The DLC coating results in a better corrosion resistance of the basic material of the mirror. PA1 Excellent running properties of the venting ring over the periphery of the mirror as a result of the very low coefficient of friction. No cold weld hereby occurs, whereby less wear occurs on mirror and venting ring. An improvement of the standing time by a factor of 10 can be expected at this moment. PA1 Improvement of the corrosion resistance of the basic material of the venting ring. PA1 A greater temperature difference (for instance 20.degree. C. instead of 10.degree. C.) is possible between venting ring and mirror, so it is possible to start sooner. In tests with a venting ring having a DLC coating the described phenomenon of jamming due to delay in heating of the venting ring was found to be considerably reduced. This improvement was the consequence of the low frictional resistance of the DLC coating. PA1 Less deposition of dirt and plastic remnants. PA1 Less pit deformation because the stamper can shift more easily beneath the resilient venting ring.
The TiN coating has resulted in a number of advantages relative to a mirror without coating. There are however still a number of drawbacks which can be mentioned which stand in the way of a higher quality of the manufactured information carrier. Some of these drawbacks are the following:
Attention is focussed in the foregoing on a mould with stamper. Use is however also made of moulds wherein the information for transferring onto the information carrier to be manufactured is arranged directly onto the relevant surface, the mirror. This mirror has an optical polished surface which is very sensitive to scratching which occurs mainly during cleaning. Due to human carelessness the mirror can become unusable due to damage during the first cleaning. During production deposits on this surface which consist of impurities such as dust and dirt and precipitation of for instance additives which evaporate out of the injected plastic. Residual particles of the plastic can become fixed on the outer periphery which result from burr-formation between the moving venting ring and the mirror.
Frictional resistance between the plastic flowing past and the mirror surface causes stress in the produced disc which has an adverse effect on the crystal-clear character of the disc and results in an undesired diffusion of the scanning laser beam during reading of the information on this information carrier.
There is the further phenomenon of cold weld between the mirror and the resilient venting ring shifting therealong. This causes damage both to the mirror and to the venting ring due to deformation of the surface of the slide fitting of both components.
The different mutually movable components of a mould are subject to mutually differing temperatures, which may change in value during successive production cycles. In respect of the necessary very small gap widths, thermal expansion of the different mould parts involved may cause problems. A mould often even has to be designed with undesirably large clearances between diverse components since, in respect of the limited possibilities of temperature control, there is otherwise the danger of a mould jamming. This problem occurs for instance in the case of a for instance resiliently disposed venting ring which is relatively shiftable co-axially and with very slight clearance relative to the mould part around which it is arranged.
This problem generally occurs in the case of a very small gap width in the transition zone between two mould components in the immediate vicinity of the mould cavity.
In respect of the above formulated problems, the invention has for its object to embody an injection mould such that it is essentially free from the described drawbacks.